1. Field of the Invention
The present invention relates to medical devices and, more particularly, to bifurcated stents and methods of making bifurcated stents for insertion within branching lumen.
2. Background of the Related Art
Stents and similar implantable medical devices, collectively referred to hereinafter as stents, are generally radially expandable endoprostheses. They are typically used to obtain and maintain the patency of the body passageway while maintaining the integrity of the passageway. Stents have provided doctors with a desirable alternative to the more invasive surgeries historically required to open obstructed passageways within the body. With the tendency being to avoid invasive surgeries, their use and range of applications has steadily increased.
Stents are tubular devices. That is, they comprise a body or wall that defines a lumen. Stents are frequently made of a thin-walled metallic or woven material and have a pattern of apertures, openings or holes defined around the circumference of the stent along most of its length. Typically, the pattern of apertures, openings or holes is configured to permit the stent to move from a contracted to an expanded position. Stents may be constructed from a variety of materials such as stainless steel, Elgiloy, nitinol, shape memory polymers, etc. The materials are typically selected for their biocompatibility among other physical characteristics that may be desirable for particular applications.
Stents are typically configured to be implanted translumenally and enlarged radially after being positioned. They may be implanted in a variety of body lumens or vessels such as within the vascular system, urinary tracts, bile ducts, etc. The stent may provide a prosthetic intralumenal wall or wall support. Some stents are particularly adapted to reinforce blood vessels and to prevent restenosis following angioplasty in the vascular system. In the case of a stenosis, a stent may provide an unobstructed conduit for blood to move through the stenotic region of the vessel. In other variations, a stent may be used to treat an aneurysm by removing the pressure on a weakened part of an artery so as to reduce the risk of embolism, or of the natural artery wall bursting.
Stents may be formed in a variety of methods. For example, a stent may be formed by etching or cutting the stent pattern from a tube or section of stent material; a sheet of stent material maybe cut or etched according to a desired stent pattern whereupon the sheet may be rolled or otherwise formed into the desired tubular or bifurcated tubular shape of the stent; one or more wires or ribbons of stent material may be braided or otherwise formed into a desired shape and pattern.
Stents are typically provided in general two configurations, self-expanding and balloon expandable, or hybrids thereof. Self-expanding stents are generally spring-like devices which are inserted in the body passageway in a contracted state within a catheter or introducer. A self-expanding stent is biased so as to expand upon release from the delivery catheter. When released, the stent reconfigures from a contracted to an expanded position. The self-expanding stent tends to increase to a final diameter dependent on the size and configuration of the stent and the elasticity of the body passageway. Self-expanding stents expand into place when unconstrained, without requiring assistance from a balloon. Balloon expandable stents require mounting over a balloon, positioning, and inflation of the balloon to expand the stent radially outward. Generally, a balloon expandable stent will include a balloon positioned within its central passage. Once the balloon expandable stent has been properly positioned, the balloon is expanded thereby expanding the stent so that the stent is urged in place against the body passageway. As indicated above, the amount of force applied is at least that necessary to maintain the patency of the body passageway. Once properly expanded, the balloon is deflated and withdrawn from the patient. Ideally, the stent will remain in place and maintain the target area of the body passageway substantially open to preserve a desired degree of the passage's function. Some stents may be characterized as hybrid stents which have some characteristics of both self-expandable and balloon expandable stents.
The balloon expandable stent, self-expanding stent, and hybrids thereof may be delivered to the target area of the body passageway by a catheter system. In such systems, the catheter or introducer typically enters the body from an access location outside the body, such as through the patient's skin, or by a “cut down” technique in which the entry blood vessel is exposed by minor surgical means. Accordingly, such systems are typically considered to be minimally invasive. In one exemplary technique, a stent may be implanted in a blood vessel at the site of a stenosis or aneurysm endolumenally. A guide wire is initially passed through the blood vessel to a position near or at the desired point of implantation. A self-expanding stent is typically provided restrained in a radially compressed configuration within a sheath or catheter. A balloon expandable stent typically does not need to be restrained and is therefore merely positioned within a sheath or catheter. The stent and associated catheter are typically configured to be received over the guideware and positioned over the guidewire at the desired location. When the catheter has been properly positioned within the blood vessel, the catheter is manipulated to cause the stent to be removed from the surrounding sheath or catheter in which it is restrained or positioned. The stent then expands or is expanded. Stent expansion may be effected by spring elasticity, balloon expansion, or by the serf-expansion of a thermally or stress-induced return of a memory material to a pre-conditioned expanded configuration. At various points during implantation, various components of the catheter system are withdrawn and typically only the stent remains after implantation.
Recently, small stents have been inserted into coronary arteries after a coronary angioplasty procedure. Coronary angioplasty is a medical procedure used to treat blocked coronary arteries as an alternative to a coronary bypass operation. It involves the insertion of a balloon catheter into the blocked artery and the inflation of the balloon to expand the size of the artery and relieve the blockage. While the procedure is often effective in opening the artery, one problem is the tendency of the artery to reclose in a process known as restenosis. If this occurs, the angioplasty procedure must be repeated which is obviously expensive and may be risky for the patient. The use of a stent after a coronary angioplasty reinforces the walls of the artery and has been shown to prevent reclosing of the artery or to at least prolong the time the artery takes to reclose.
Within the vasculature however, it is not uncommon for stenoses to form at any of a wide variety of vessel bifurcations. A bifurcation is an area of the vasculature or other portion of the body where a first (or parent) vessel is bifurcated into two or more branch vessels. Bifurcations exist within the body in a wide variety of configurations, angles, and vessel diameters. Where a stenotic lesion or lesions form at such a bifurcation, the lesion(s) can affect only one of the vessels (i.e., either of the branch vessels or the parent vessel) two of the vessels, or all three vessels.
Originally, stents shared the common design of being mono-tubular and thus, are best suited to be delivered and implanted in-line in the body passageway. However, mono-tubular stents are not optimal for use at a bifurcation body passageway or about a side branch of a body passageway. When implanted, mono-tubular stents may shield side branches emanating from the target area of the body passageway. In these cases, there is an increased risk of closure of one of the side branches or arm of the bifurcation and, at a minimum, the increased resistance to the movement of fluid through the obscured branch or arm.
To accommodate branching and bifurcations, some techniques have utilized individual stents in each branch of the bifurcated body passageway. However, this approach is fraught with at least two significant problems. First, implantation of three individual stents, together with the expansive forces generated upon implantation results in subjecting the central walls of the bifurcated body passageway to undue stress which may lead to post-procedural complications. Second, since the central walls of the bifurcated body passageway are not supported by the individual stents, this area of the passageway is left substantially unprotected and susceptible to blockage. An array of bifurcated stents have also been developed for application at bifurcations or branches in lumen. However, these stents frequently lack flexibility at the junction of the three branches and are complicated to manufacture.
Thus, there remains a need for a bifurcated stent that may be utilized in a wide variety of vessel bifurcations and branches, without requiring that the stent be extensively modified prior to implantation, and to provide a stent that is capable of supporting the a vessel bifurcation without inhibiting fluid flow.